Glossary P

P.D. Pump

  • Positive displacement pump. It can pump a high pressure or head, but at a low volume.

P.V. Factor

  • An attempt to correlate the relationship between the pressure and the velocity at the mechanical seal faces. Unfortunately carbon graphite is sensitive to P, but not to V so the correlation does not work too well.


  • The soft rings that mechanical seal replace to stop leakage. Packing must leak because it works on the theory of a series of pressure drops to reduce the stuffing box pressure to the point where the leakage is acceptable. A minimum of five rings of packing is required to do this.

Parallel operation

  • The pumps are discharging to a common header. It is important that the impeller speed and outside diameters be the same or one of the pumps will throttle the other.


  • A metric unit for pressure. 100 kPa = one atmosphere.


  • The protective oxide layer (ceramic) has formed on the metal surface.


  • A measure of the acidity or the alkalinity of a fluid. The scale ranges from 0 (acid) to 14 (alkali) with 7 considered neutral.


  • Same as the specifications.

Pipe strain

  • The strain on the pump volute caused by the piping. It will cause excessive mechanical seal movement and can cause contact between rotating and stationary pump and seal components.


  • Surface voids caused by corrosion, erosion or cavitation.

Plasma spray

  • A method of putting a hard coating on a base material. Not recommended for mechanical seal faces.

Plastic range

  • If you stress metal enough, it stretches through the elastic range, its yield point and ends up in the plastic range where it loses its memory.

Positive displacement pump

  • Called a PD pump. Gear, sliding vane, progressive cavity, lobe etc. the capacity determined by the pump speed. The maximum head is determined by the horsepower available and the casing strength.

Power end

  • The end of the pump that attaches to the power source and does not get wet from the pumpage. The bearings are in this part.

Power factor

  • A measure of how the voltage leads or lags the amperage.

Plastic range

  • The metal has been stressed beyond its yield point and will not return to its original shape.

Precision bearing

  • Ball or roller bearing as opposed to a sleeve or babbitt bearing.

Pressure drop

  • Referring to the loss of pressure from the outside to the inside of the mechanical seal faces or across the individual rings of packing.

Press fit

  • The best way to insert a carbon/graphite face into a metal holder. The carbon will shear to conform to the holder’s irregular shape. Usually done with an arbor press.

Pressure gradient

  • The pressure drop between the seal faces. Usually illustrated by a wedge.

Pressure head

  • The pump head exerted by atmospheric pressure or any additional pressure that might be in the vessel.

Pump curve

  • A diagram supplied by the pump manufacture to describe the relationship between the head and the capacity of a particular pump using various size impellers. The curve also include information about efficiency, horse power consumption, N.P.S.H. required, etc.

Pump out vane

  • Located behind the impeller shroud in some impeller designs to lower stuffing box pressure. Should no be used in hot well condensate pumps or any pump running with a negative stuffing box pressure.

Pumping ring

  • Used with a convection system to get circulation between two mechanical seals. Absolutely necessary if oil is used as a barrier fluid because of oil’s poor specific heat..

Pusher seal

  • A design that has a spring loaded dynamic elastomer or rubber like part. A very poor design that should be avoided.


  • On February 09, 2018